Lecture 2 Flashcards
(40 cards)
The Cenozoic Era (65Ma -present) –‘age of mammals’
- final stages of Pangaea break-up: Australia separated from Antarctica and Greenland from North America. The Atlantic Ocean continued to grow
- the collision of India and Africa with Asia and Europe formed the Alpine-Himalayan chain.
- first hominid is ~6-7Ma
- Neanderthals is ~350,000 years ago
- modern human is around ~200,000 years ago
The Cenozoic Era (65Ma -present) –> 65 million years of climate change
- for much of our recent era, the earth has been cooling
- Antarctica ice sheets started forming 1.5 Ma ago
- North American ice sheets started 5 Ma ago
Glacial-interglacial cycles
- ice core records (800,000 yrs):
9 interglacials
9 glaciations - largest changes = 100,000 year cycles
- smaller cycles follow 20 and 40 k cycles
- dust, temperature, CO2, methane (CH4) and N2O show similar trends
The last glacial maximum (LGM)
- Wisconsin glaciation
- lower concentrations in greenhouse gases
- large continental ice sheets
- ended 18,000 yrs ago
- lower sea levels (120 m)
The end of the earth
- will probably occur 5 Ga in the future, when the Sun runs out of nuclear fuel, and transforms briefly into a red giant, whose diameter will approach that of the earth’s orbit; as the expanding sun’s surface will approach the earth, the planet will vaporize.
- this will be the ultimate global change
The Tohoku earthquake (2011): shaking Japan’s trust in nuclear power
- earthquake triggered a tsunami
- waves swept over sea wall and floated Fukushima Daiichi nuclear power plant
- caused a nuclear meltdown
- thousands died and billions of dollars in damage
- caused public to become more wary of nuclear power
Some basics
Matter:
- everything around you (and in the universe) that has mass
- made up of atoms and molecules
Different atoms are called elements:
Elements: different number of protons and electrons
Molecules are made up of two or more atoms
The earth is one large mixture of molecules in gases, liquids and solids
Some basics
- Isotopes = atoms with differing numbers of neutrons
- Mass number =the combined number of protons and neutrons
- isotopes of an element behave differently
- some isotopes are radioactive and decay until they become non-radioactive stable isotopes.
Some basics
Radioactive decay:
- some isotopes are unstable
- they undergo radioactive decay and are converted to a different element
- half-life: the amount of time necessary to reduce the number of atoms by 50% from the original number.
- different radioscopes have different half-lives
Geologic time
Time: sequence of events
The age of events can be specified as relative age or as numerical (absolute) age
Absolute ages -impossible for early geologists (until the 20th century)
Early geologists were able to determine only the relative ages of the various parts of the earth . For this they were using several principle: uniformitarianism, original horizontality, superposition, original continuity, cross-cutting relations. (Establishing the relative age-an example)
Geologic time scale
The construction of the geologic time scale was initially based on relative age determinations of sedimentary rock units (by using the principles of stratigraphy) and on correlations of widely separated units using ‘Fossils’.
Until the 20th century, only the relative ages were unknown
Geologic time: age for the earth
In 1658 archbishop ussher of Ireland gives an age of about 5600 years, based on the study of Old Testament
In the late 18th century, James Hutton introduces the principle of Uniformitarianism:
(1) Earth’s history is a constant cycle of deposition, burial, uplift, erosion & re-deposition
(2) same processes we observe today operated in the past .
Geologic time: age for the earth (continued)
In the 19th century:
Charles Darwin estimated the age of the Earth, based on how long it would have taken to erode a large valley in the south of England: ~300,000,000 years
William Thomson (Lord Kelvin) estimated based on the cooling of the Earth from an initial high temperature to the present state: ~100,000,000 years –> however, he did not include the heat from radioactive decay
Geologic time: age for the earth (continued)
In 1896 Henri Becquerel discovers Radioactivity.
Radioactive dating of the Earth:
‘Daughter ‘ products form from ‘parents’ at characteristic rates of decay (half-lives)
Lead-lead geochronology: …
1956: Claire Patterson 4.55 billion years old
Geological time scale
Make sure to know the table of pg15, the time scale correlates with each event.
Basic Geology
Lithosphere: upper 100km (include crust and upper mantle)
Athenosphere: warmer ductile layer of the mantle that ‘flows’ part of upper mantle.
Mantle: surrounds the core, thick layer of rock
Core: the planet’s center, consisting mostly of iron, solid in the inner core and molten in the outer core.
Plate Tectonics
- movement of lithospheric plates
- fifteen major tectonic plates
- where two plates meet:
(1) intense geological activity
(2) Earthquakes, volcanoes
(3) Mid Ocean ridges
Two types of crust
Continental crust:
(1) less dense
(2) thicker
(3) older
Oceanic crust:
(1) denser
(2) thinner
(3) younger
The driving force is the heat energy from the mantle & core derived from radioactive decay
Plate Boundaries: 3 types
A. Divergent
B. Convergent
C. Transform
Divergent (spreading) plate boundaries
- their surface expression is a mild-ocean ridge
- associated with high heat flow, which reflects rising of magma
- newly formed oceanic crust moves away from the ridge axis, then more magma rises, and more crust forms, and so on…
- since new crust is made are mid-ocean ridges, the age of the ocean floor increases progressively from the ridge axis.
Convergent plate boundaries
- the region where the oceanic crust returns to the mantle are called subduction zones. They mark convergent plate boundaries.
- the surface expression of a subduction zone is a trench and a volcanic arc.
- subduction zones have intense seismic activity.
- collisions lead to the formation of major mountain ranges (e.g Alps, Appalachians, Andes, Cascades…)
Convergence of two oceanic plate leads to a volcanic island arc
Convergence between an oceanic and a continental plate leads to a continental volcanic arc. –> ring of fire.
Deepest ocean trench is the Mariana Trench at 11,022 meters (36,161 feet)
Continental lithosphere cannot subduction because it is too buoyant.
When two blocks of continental lithosphere converge, a collision occurs- neither block can be subducted because of their buoyancy.
Transform Faults
- Often Link segments of the ridge system, some transform boundaries link trenches, other link a trench to a ridge segment.
- No new plate forms and no old plate is consumed at a transform boundary.
- famous example of a transform plate boundary: The San Andreas fault.
Hot Spots - intraplate volcanoes
Global hotspot locations…
Volcanoes
- Movement of tectonic plates on hot soft rock (anthenosphere)
- Heat melts rock –> magma rises
- Lava: magma at surface
- Happens at:
(1) subduction zones
(2) spreading centers
(3) Hot spots
